There is known a noble metal fine particle dispersed liquid in which noble metal fine particles are dispersed stably with protective colloids. When the noble metal fine particles are taken out from the noble metal fine particle dispersed liquid and used for, for example, producing a conductive material, the noble metal fine particles need to be withdrawn effectively. Usually, when noble metal fine particles need to be withdrawn effectively from a noble metal fine particle dispersed liquid containing protective colloids, a protective colloid remover is added to this noble metal fine particle dispersed liquid to remove the protective colloids to some extent, and a flocculant further is added thereto to aggregate the noble metal fine particles. The following materials can be mentioned as examples of the protective colloid, the protective colloid remover, and the flocculant.
(Protective Colloid)
(1) Protein protective colloid: gelatin, gum arabic, casein, a casein compound
(2) Natural polymer: starch, dextrin, agar, sodium alginate
(3) Cellulose protective colloid: hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, modified cellulose
(4) Synthetic polymer protective colloid such as the following.
Vinyl protective colloid: polyvinyl alcohol, polyvinyl pyrrolidone
Acrylic protective colloid: sodium polyacrylate, ammonium polyacrylate
Others: polyethylene glycol, polypropylene glycol
(Protective Colloid Remover)
(1) Protein degraing enzyme (such as serine protease)
(2) Natural polymer degrading enzyme (such as amylase that is a starch degrading enzyme)
(3) Cellulose degrading enzyme (such as cellulase)
(4) Organic solvent (such as formamide), acid, alkali
The numbers indicating the above-mentioned protective colloid removers correspond respectively to the numbers indicating the protective colloids to be removed (for example, the organic solvent listed in (4) is used as the remover for the synthetic polymer protective colloid).
(Flocculant for Aggregating Noble Metal Fine Particles)
(1) Anionic flocculant (such as a product obtained by partial hydrolysis of polyacrylamide)
(2) Cationic flocculant (such as polyacrylamide, and dimethylaminoethyl methacrylate)
(3) Amphoteric flocculant (such as an alkylamino(meth)acrylate quaternary salt-acrylamide-acrylate copolymer)
There is also known a method in which an acid is added as a flocculant and the pH thereof is adjusted to accelerate the aggregation.
Noble metal fine particles withdrawn from a noble metal fine particle dispersed liquid and dried into a powdery state are redispersed in a dispersion medium, such as a solvent and a paste, and the resultant can be used as a coating material, a conductive paste, etc.
However, in the case of the noble metal fine particle powder withdrawn after the protective colloids have been removed to some extent, the noble metal fine particles have a lower dispersibility (redispersibility) at the time when being redispersed in a dispersion medium such as a solvent. Moreover, the withdrawing method in which a protective colloid remover is added to a noble metal fine particle dispersed liquid and a flocculant further is added thereto requires complicated work.
There is also known a withdrawing method in which a complexing agent is added to a noble metal fine particle dispersed liquid. However, when a noble metal fine particle powder is withdrawn by this method, it is likely that the adsorbed protective colloids are desorbed and the effect of enhancing the redispersibility by the protective colloids is lowered. Moreover, in this method, the added complexing agent is an impurity that worsens the redispersibility.
A conventional method for withdrawing noble metal fine particles from a noble metal fine particle dispersed liquid containing protective colloids is disclosed in Patent Literature 1, for example.